Yeah, this is a good place to ask!
As you guessed, hold is a fairly standard PbtA thing that we forgot to clarify in the playkit (it'll definitely be in the final rules/future playkits).
Hold works the same as any other effect triggered by a move (for example pick 2), however the main difference is that hold doesn't have to be spend immediately.
For example, the read a person move lets you hold 1 or 3, which you can spend to ask questions from a list. You could spend all of that hold immediately to ask those questions, or spend them as you work your way through a conversation.
I tend to imagine it as having a stack of tokens that I can spend to trigger the effects of a move when it makes sense to.
An important note is that you can only spend hold on the move it was gained from (unless otherwise specified). And hold only applies to that particular use of the move - if I used read a person on Sparrow, I couldn't spend hold to read a different person (I'd have to roll again). It can be useful to note it down sometimes, if you're not using it all right away.
Hope this all makes sense!